Probing diffusion of single nanoparticles at water-oil interfaces

Area of Science:

• Physical Chemistry
• Materials Science
• Nanotechnology

Background:

• Understanding nanoparticle behavior at interfaces is crucial for applications in areas like drug delivery and materials assembly.
• Liquid-liquid interfaces present unique physical environments that can alter nanoparticle dynamics.

Purpose of the Study:

• To investigate the diffusion of nanoparticles at a water-alkane interface.
• To quantify the effect of nanoparticle size and surface chemistry (hydrophilic vs. hydrophobic) on interfacial diffusion.
• To explore the influence of the viscosities of the two liquid phases on nanoparticle interfacial diffusion.

Main Methods:

• Utilizing fluorescence correlation spectroscopy (FCS) to track nanoparticle movement.
• Employing quantum dots (QDs) of varying sizes (5, 8, and 11 nm) with both hydrophilic and hydrophobic surface modifications.
• Conducting experiments at the water-decane interface, chosen for its similar phase viscosities.

Main Results:

• A significant slowdown in nanoparticle diffusion was observed at the water-alkane interface compared to bulk diffusion.
• The diffusion slowdown effect was most pronounced when the viscosities of the water and decane phases were similar.
• Interfacial diffusion coefficients were reduced by approximately 1.5 times for hydrophilic QDs and 2 times for hydrophobic QDs relative to their bulk diffusion values.

Conclusions:

• Nanoparticle diffusion is hindered at liquid-liquid interfaces, with the degree of hindrance dependent on particle properties and interfacial conditions.
• The matching of viscosities between the two liquid phases amplifies the interfacial diffusion slowdown.
• Surface hydrophobicity plays a role, with hydrophobic nanoparticles experiencing a greater reduction in diffusion at the interface.